The strong reaction Fuyubachi has in the parking lot isn't because he's having an attack with no medication to take for it...
... it's because he's finally realized, he and the other two are dead. It's a sunny day, and everyone else in the park is casting a shadow; the three of them are not. Earlier in the episode, one of their suicide attempts succeeded in bringing about their deaths, but they didn't know it, and continued trying to kill themselves.
There are clues beforehand that such a thing is possible...
Namely, the man they saw who jumped in front of the train, and whom Zebra then saw walking out of the crowd. Everyone's reactions to the death indicate that it left him in a horrible state - so bad that our three protagonists decide that even if they want to die, they don't want to do it in a way that leaves you looking like that.
Yet the man Zebra sees is moving under his own power, and while he's bloody, and complaining about how it hurts, it's nothing like the utter mangling one would expect. More to the point, no one else in the crowd seems to see the man. He is a ghost, who only thinks he's still alive - and the only reason Zebra sees him, presumably, is because Zebra is also a ghost.
If you re-watch the episode knowing what Fuyubachi realizes at the end...
It seems that our protagonists must have died in the building that was being demolished. When Zebra and Fuyubachi run away from Kamome, not wanting her to die along with them, they have shadows; when they are going to the train station, they cast no shadows on the ground, but a passing bike rider casts a very visible shadow.
Ironically, it means that their deaths probably weren't even a result of their suicide attempts, except indirectly. Fuyubachi and Zebra both take pills and try to inhale the carbon monoxide inside the building, but Kamome shows up later than that, and still dies; it suggests that they were killed when the building was demolished with them inside, something they didn't plan for.
It also explains the reactions they get from others...
... which is mostly not being noticed at all, because the living can't see them. The exceptions are:
Shounen Bat (Lil' Slugger), who can see them but is presumably a little freaked that dead people are begging him to kill them, and ...
The teenage girls at the end, who are shocked because Fuyubachi, Zebra and Kamome's photobombing turned the picture they took into a "ghost photo".
